The two belt, one alternator mystery

My Volvo has two alternator belts (picture below). Bizarrely, the belts don't tension evenly despite being the same size and routed over the same pulley pathway. The belt closest to the engine is slacker than the one nearest the radiator. I end up having the front belt slightly over-tight and the rear belt slightly looser than desired. No detected pulley play or bearing noise. Thoughts?

Doesn't sound like its a problem, but if it became one, and if this guy isn't talking crap (seems to be some argument about that) then maybe you could swap your two belts for a single one of his recommended high performance industrial aramid fibre ones.

Wish I had thought of that. It's more difficult than it looks to get them on/off, particularly getting the belt to transition from the front to rear pathway. When I saw what was happening I took them both off and visually compared them, but didn't think to swap them over to see if the problem was the pathway or the belt. Just assumed it wasn't the belt as they were both new, from the same manufacturer, had the same measurements printed on them and looked the same side-by-side.

After you mentioned this I noticed that the irrigation rigs in the ricefields around the campus use a double belt to link the pump and the single-cylinder-Chinese-Yanmar-diesel-knock-off. Probably taking more power than your alternator for long periods, but doesn't seem to cause any problem.

Over time it might be self-compensating, the tighter belt wearing more so they even-out.